Oxide superconducting device having uniform oxygen concentration

ABSTRACT

A superconducting device has an oxide superconducting material with a passivation or blocking film formed on its surface. The film helps to maintain a uniform oxygen concentration of the superconducting material through its thickness. The superconducting material is thus superconductive throughout its cross-section, and particularly in the vicinity of the surface bearing the film.

This is a divisional application of U.S. application Ser. No.07/557,705, filed Jul. 25, 1990, now U.S. Pat. No. 5,137,868, which inturn is a divisional of U.S. application Ser. No. 07/187,044, filed Apr.27, 1988, now U.S. Pat. No. 4,959,345.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for preparing a solid devicefor use as an oxide superconducting material, and more particularly to amethod for preparing a solid device, the surface of which is utilizedfor oxide superconducting material wherein an important improvement isimparted to the properties of the material at the surface or portionclose to the surface, to provide a highly reliablesurface-utilizing-device.

2. Description of the Related Art

Recently, considerable attention has been directed toward oxidesuperconducting materials. This began with the development of aBa-La-Cu-O type of oxide superconducting material in the IBM researchlaboratories in Zurich, Switzerland. In addition to this, an yttriumtype of oxide superconducting material is also known, which has providedthe obvious possibility for the practical application of a solid deviceat the temperature of liquid nitrogen.

On the other hand, superconducting materials using metals such as Nb₃ Gehave been well known conventionally. Trials have been conducted infabricating solid devices such as the Josephson element using this metalsuperconducting material.

After a dozen years of research, a Josephson device using this metal isclose to being realized in practice. However, the temperature of thissuperconducting material at which the electrical resistance becomes zero(which is hereinafter referred to as Tco) is extremely low, that is 23°K., so that liquid helium must be used for cooling. This means thatpractical utility of such a device is doubtful.

With a superconducting material made of this metal, the components onboth the surface and in the bulk of the material can be made completelyuniform because all the material is metal.

On the other hand, when the characteristics of the oxide superconductingmaterial which has been attracting so much attention recently areexamined, a deterioration of the characteristics (lowering ofreliability) is observed at the surface or portion close to the surface(roughly 200 Å deep), in comparison with the bulk of the material.

It has been possible to prove experimentally that the reason for this isthat the oxygen in the oxide superconducting material can be easilydriven off.

Further, when observed with an electron microscope, an empty columnarstructure is seen with an inner diameter of 10 Å to 500 Å, and usually20 Å to 50 Å in the oxide superconducting material, and in other words,the oxide superconducting material is found to be a multiporous materialhaving indented portions in micro structure. For this reason the totalarea at the surface or portion close to the surface is extremely large,and when this oxide superconducting material is placed in a vacuum, theoxygen is broken loose as if absorbed gas was driven off.

The basic problem is determined that whether the material hassuperconducting characteristics or simply normal conductingcharacteristics is dependent on whether the oxygen is present in idealquantities or is deficient.

SUMMARY OF THE INVENTION

An object of the present invention is to provide, with due considerationto the drawbacks of such conventional devices, a method for preparing asuperconducting device which is kept superconductive at the surface orportion close to the surface of the oxide superconducting material.

This is accomplished in the present invention by the provision of ablocking film (passivation film), which is uniformly coated over thespaces or micro-holes in the surface portion of the superconductingmaterial, to prevent the removal of oxygen from that surface.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features, and advantages of the presentinvention will become more apparent from the following description ofthe preferred embodiments taken in conjunction with the accompanyingdrawings, in which:

FIG. 1(A) to FIG. 1(E) are a diagram indicating the method of preparingthe superconducting device of the present invention and showing thedistribution of the oxygen concentration.

FIG. 2(A) and FIG. 2(B) are an enlarged sectional drawing of asuperconducting material for implementing the method of the presentinvention.

FIG. 3 is an enlarged sectional view of an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the embodiments of the present invention, a blocking film orpassivation film is uniformly coated over the spaces or micro-holes inthe surface portion of the superconducting material, to prevent theremoval of oxygen from that portion. Subsequently, a means is added bywhich the amount of oxygen in the inside surfaces of the superconductingmaterial which tend to become oxygen deficient, can be preciselycontrolled. The superconducting material therefore has the sameconductivity characteristics at the surface portion as at the internalportion.

In the present invention, a film is formed on the surface of thesuperconducting material at a thickness of 10 Å to 2 μm using an opticalCVD method superior in stepped coverage, which is a method of exciting areactive gas using ultraviolet light for coating a film onto a filmforming surface. In particular, if this film is to e an insulated orhalf-insulated film for use in a Josephson element, it is formed at athickness of 10 Å to 50 Å. Also, in the case where it is to be used as apassivation film, it is formed in a thickness of from 1000 Å to 2 μm.

After this, by means of methods such as the ion injection method or hotoxidation method, oxygen is added onto the surface or portion close tothe surface, and the entire body is heat treated, so that the addedoxygen is positioned in the appropriate atom location. In addition, thisfilm is converted by heat treatment to a highly dense insulatingmaterial to provide a more complete blocking layer. This film isoxidized on a metal or semiconductor and is formed to function as aninsulating film. Further, by solid phase to solid phase diffusion of theoxygen in this film, that is diffusion of the oxygen from a solid filminto another ceramic which is solid, the oxygen concentration in theregion at the surface or close to it, generally at a depth of about 200Å, can be appropriately controlled.

The films used for this purpose may be insulating films such as siliconnitride, aluminum nitride, oxidized aluminum, oxidized tantalum,oxidized titanium and the like.

In addition, a metal or semiconductor which becomes an oxidizedinsulating film after oxidizing treatment can be used as this film.Specific examples are, in a metal, aluminum titanium, copper, barium,yttrium, or in a semiconductor, silicon or germanium. These materials,by oxidation, can be made into aluminum oxide, titanium oxide, tantalumoxide, copper oxide, barium oxide, and yttrium oxide. Also, silicon canbe converted into silicon oxide, and germanium into germanium oxide.

With the present invention, an oxide superconducting material formedinto tablets, or a superconducting material formed into a thin film canbe used. Especially with the use of a thin film structure, the screenprinting method, sputtering method, MBE (molecular beam epitaxial)method, CVD (chemical vapor deposition) method, optical CVD method, andthe like can be used.

One example of an oxidized superconducting material used in the presentinvention can be generally represented as (A_(1-x) B_(x))_(y) Cu_(z)O_(w), where x=0 to 1, y=2.0 to 4.0 or, preferably, 2.5 to 3.5, z=1.0 to4.0 or, preferably, 1.5 to 3.5, and w=4.0 to 10.0 or, preferably, 6.0 to8.0. A is one or a plurality of elements which can be selected from thegroup of Y (yttrium), Gd (gadolinium), Yb (ytterbium), Eu (europium), Tb(terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Lu(lutetium), Sc (scandium), and other elements in Group III of thePeriodic Table. B can be selected from among elements in Group IIa ofthe Periodic Table, such as Ra (radium), Ba (barium), Sr (strontium), Ca(calcium), Mg (magnesim), and Be (beryllium). In particular, as aspecific example, (YBa₂)Cu₃ O₆₋₈ can be used. In addition, lanthanideelements or actinide elements in the Periodic Table other than thoseoutlined above can be used as A.

In the present invention, when the insulating film is of a thicknesscapable of causing a tunnel current of 5 Å to 50 Å to flow, anothersuperconducting material 3, as shown in FIG. 3 can be positioned on theupper surface of this insulating film to provide a Josephson elementstructure.

In addition, it can also be used as a passivation film, that is a filmto prevent deterioration, at a thickness of from 1000 Å to 2 μm.

Specifically, after the film is formed on the oxide superconductingmaterial, oxygen can be added, or, added oxygen can be positioned in anappropriate location, by use of a heat treatment at from 300° C. to 900°C., for example 500° C., for 0.5 to 20 hours, for example, 3 hours, inan atmosphere of inert gas, air, or oxygen, so that the surface of thematerial or the portion close to the surface can be superconductive.

As a result, the oxygen concentration of this surface can be maintainedin an ideal status when maintained at the temperature of liquidnitrogen. Specifically, a passivation film can be created.

In this way, the problem which has existed up until the present time,that is, the problem that the superconducting state close to the surfaceof an oxide superconducting material disappears for unknown causes, iscorrected, and the superconductive state of the surface can beeffectively utilized with long-term stability.

As a result, the surface utilizing device, especially a Josephsonelement, can be activated with long term stability and high reliability.

First Example

Now referring to FIG. 1(A) to FIG. 1(E), the structure of a firstexample of the present invention and the characteristics of the relativedistribution of the concentration of oxygen in this embodiment areshown.

FIG. 1 (A) shows a superconducting material, for example YBa₂ Cu₃ O₆₋₈.The copper component may be 3 or less. The starting material (FIG. 1(A)(1)) was formed from such a superconducting material in tablet orthin film form, having a monocrystalline or polycrystalline structure.

When this material was placed in a vacuum in a vacuum device, the oxygenin the area close to the surface (1') was removed, so that thedeterioration of electrical characteristics occurred in a depth range upto about 200 Å.

When this surface was observed through an electron microscope, deepspaces or micro-holes were seen to be formed from the surface to theinterior of the material, as shown in FIG. 2 (A). These spaces have aninternal diameter of 10 Å to 500 Å, and usually from 20 Å to 50 Å. Theoxygen density corresponding to FIG. 1 (A) is shown in FIG. 1 (D). And,it has been confirmed the oxygen at the surface or close to the surfacecan be easily removed. A region 1 in the diagram had a normal oxygenconcentration, while there was a deficiency of oxygen in a region 1'.The depth of the region 1' with a deficiency of oxygen was 50 Å to 2000Å. This depth varied depending on the type, structure, and density ofthe superconducting material, but was generally about 200 Å.

On the surface of this material, a silicon nitride film, a silicon oxidefilm, or an aluminum film was formed to a depth of 5 Å to 50 Å, forexample, 20 Å, by the CVD method, in which a reactive gas is opticallyexcited using ultraviolet light or a laser beam, so that a film isformed on the treated surface. The silicon nitride was formed at atemperature of 250° C. and a pressure of 10 torr, from the followingreaction:

    3Si.sub.2 H.sub.6 +8NH.sub.3 →2Si.sub.3 N.sub.4 +21H.sub.2

In this way, it was possible to form a film so that the inside of thespaces was adequately coated. In addition to this treatment, ioninjection was also carried out. A lower accelerating voltage of 10 KV to30 KV was applied and doping was carried out, so that the oxygenconcentration became uniform at a concentration of 1×10¹⁷ cm⁻³ to 1×10²¹cm⁻³.

Heat treatment was applied to the whole body in an atmosphere of oxygenat 300° C. to 900° C., for example 500° C. for about 5 hours.

As a result of this heat treatment, it was possible to impart the sameoxygen density to the surface portion as in the internal portion asshown in FIG. 1 (E).

A sample of this embodiment of the present invention was removed fromthe heat treatment condition and once more stored in a vacuum. Ablocking layer 3 formed in this manner on the surface or portion closeto the surface of the superconducting material made it possible toproduce a highly reliable device, with no oxygen deficiency in thatportion.

This insulating film was extremely effective as a passivation film.

Second Example

In a second example of the present invention silicon oxide was used forthe film.

The silicon oxide was formed at a temperature of 200° C. usingultraviolet light at 185 nm and a pressure of 20 torr, implementing aphotochemical reaction as indicated in the following equation:

    SiH.sub.4 +4N.sub.2 O→SiO.sub.2 +4N.sub.2 +2H.sub.2 O

The superconducting material was the same as in the first example.Subsequently, a heat treatment in oxygen at 450° C. was carried out anda suitable oxygen concentration obtained.

Third Example

In a third example of the present invention, metallic aluminum was usedfor the film.

The aluminum film was formed at a temperature of 250° C. and a pressureof 3 torr, using a photo-CVD process at a wavelength of 185 nm,implementing a photochemical reaction as indicated in the followingequation:

    2Al(CH.sub.3).sub.3 +3H.sub.2 →2Al+6CH.sub.4

Subsequently, the material was annealed in oxygen at 500° C. for 3 to 10hours, and, as in the first example, the aluminum on the surface wasconverted to alumina, and the concentration of oxygen was optimizedthroughout the superconducting material.

An oxide superconducting material is used in the present invention, andthe surface, when examined with a electron microscope, is seen to have alarge number of micro-holes or spaces. It is necessary to fill theinside of the spaces or the micro-holes with a solid material to have ahigh degree of reliability. A film produced by the vacuum evaporationmethod, hot CVD method, sputtering method and the like cannot cover theinternal surface. However, when the photo-CVD method is used in thepresent invention, an extremely superior coating is possible, so that anextremely minute coating can be obtained on the top surface of theporous substrate material used. In addition, by making this coating moredense, or converting to an oxidized insulating material, a more perfectstate can be obtained, and at the same time it is possible to fill themicroholes or spaces. In addition, this method by which an improved,dence, superconducting material is obtained is extremely effectivebecause the manufacturing process is very easy.

In the present invention the term "oxide superconducting material" isused, wherein it is clear that in the technical concept of the presentinvention, the crystal structure may be either monocrystalline orpolycrystalline. In particular, in the case of a monocrystallinestructure, epitaxial growth may occur on the substrate for use as thesuperconducting material.

In the present examples, after the film has been formed, oxygen isinjected into the superconducting material by ion injection. However, itis possible to add oxygen to the surface or portion close to the surfaceof the superconducting material in advance by the ion injection methodor the like, and to form the film afterward, before effectivelypositioning the added oxygen in the appropriate atom location by a hotoxidation process when fabricating the superconducting material.

What is claimed is:
 1. A superconducting device comprising an oxidesuperconducting material and a passivation film formed on a surface ofsaid superconducting material, wherein said material exhibitssuperconductivity at said surface or in its vicinity as well as in abulk portion of said material.
 2. The device of claim 1 wherein saidpassivation film is 10 Å-2 μthick.
 3. The device of claim 1 wherein saidsuperconducting material is represented by a general formula:

    (A.sub.1-x B.sub.x).sub.y Cu.sub.z O.sub.w,

where 0<x<1, 2<y<4, 1<z<4 and 4<w<10, A is one or more elements selectedfrom the group consisting of Sc, Y, lanthanides and actinides and B isone or more elements selected from the group consisting of Be, Mg, Ca,Sr, Ba and Ra.
 4. A superconducting device, comprising an oxidesuperconducting material having a bulk portion and a surface, apassivation film formed on said surface of said superconductingmaterial, said passivation film comprising a material selected from thegroup consisting of aluminum oxide, titanium oxide, copper oxide, bariumoxide, yttrium oxide, silicon oxide, germanium oxide, silicon nitrideand germanium nitride,wherein said superconducting material exhibitssuperconductivity at said surface or in its vicinity as well as in saidbulk portion of said material.
 5. A superconducting device comprising anoxide superconducting material and a passivation film formed on asurface of said superconducting material, wherein an oxygenconcentration of said material is uniform through its thickness.
 6. Asuperconducting device comprising a pair of superconducting filmscomprising an oxide superconducting material and a blocking filminterposed between said pair, said blocking film capable of passing atunneling current therethrough, wherein a concentration of oxygen insaid superconducting material is uniform through thickness of saidsuperconducting films.
 7. The device of claim 6 wherein said blockingfilm is 10-50 Å thick.
 8. A superconducting device comprising a pair ofsuperconducting films made of an oxide superconducting material and ablocking film interposed between said pair, said blocking film capableof passing a tunneling current therethrough, wherein said blocking filmcomprises a material selected from the group consisting of aluminumoxide, titanium oxide, copper oxide, barium oxide, yttrium oxide,silicon oxide, germanium oxide, silicon nitride and germanium nitride.9. The device of claim 8 wherein said blocking layer is 10-50 Å thick.